Image forming apparatus

ABSTRACT

An image forming apparatus includes a feeder unit configured to feed one or more sheets held in a sheet container, and a sheet sensor configured to detect whether or not there are any sheets in the sheet container. When a sheet is to be fed to an image-forming unit, the feeder unit preliminarily conveys the sheet in a sheet feeding direction by a predetermined distance from a held position in the sheet container so that a trailing end of the sheet passes a detecting position of the sheet sensor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to image forming apparatuses that are capable of forming images on sheets.

2. Description of the Related Art

FIG. 7 illustrates a conventional image forming apparatus. The image forming apparatus includes photoconductive drums 100 a to 100 d on which yellow (Y), magenta (M), cyan (C), and black (Bk) toner images can be formed, respectively. The toner images formed on the photoconductive drums 100 a to 100 d are primarily transferred onto an intermediate transfer belt 12 in sequence so as to become a full-color toner image. Developing portions 110 a to 110 d are provided for developing electrostatic latent images formed on the photoconductive drums 100 a to 100 d using toner so as to form the toner images on the photoconductive drums 100 a to 100 d.

A sheet cassette 40 provided in the image forming apparatus is capable of holding sheets P therein. When one of the sheets P contained in the sheet cassette 40 is fed by a feed roller 15, a resist unit 16 conveys the sheet P towards a secondary transfer unit 13 at a timing at which the full-color toner image on the intermediate transfer belt 12 and a leading end of the sheet P can be aligned with each other. At the secondary transfer unit 13, the full-color toner image on the intermediate transfer belt 12 is secondarily transferred onto the sheet P. Subsequently, the sheet P having the full-color toner image transferred thereon is conveyed to a fixing unit 14 where the toner image is fixed on the sheet P.

The sheet cassette 40 is provided with a trailing-end regulation plate 42 which is movable in a direction indicated by an arrow A. When the sheets P are placed in the sheet cassette 40, the trailing-end regulation plate 42 is moved in accordance with the size of the sheets P so that the position of a trailing end of the sheets P can be regulated.

The sheet cassette 40 is also provided with a sheet sensor 41 that is configured to detect whether there are any sheets P in the sheet cassette 40. In a case where the sheet sensor 41 determines that the sheet cassette 40 is empty, a controller (not shown) stops the image forming operation, or a display portion gives an indication that there are no sheets in the sheet cassette 40.

Regardless of the sheet size, the sheet sensor 41 must have an ability to properly detect whether or not there are any sheets in the sheet cassette 40. A known technique for achieving this is by setting the sheet sensor 41 in an area of the sheet cassette 40 near the leading end of the sheets P in the sheet feeding direction (for example, see Japanese Patent Laid-Open No. 8-278724).

FIGS. 8A and 8B show an example in which the sheet sensor 41 is disposed in an area of the sheet cassette 40 near the leading end of the sheets P in the sheet feeding direction. The sheet sensor 41 of this example includes, for example, an optical sheet sensor portion 41 a and a flag 41 b. The sheet sensor portion 41 a is provided with a light emitting component (not shown) and a light receiving component that can receive light emitted from the light emitting component. The sheet cassette 40 is provided with an intermediate plate 43 that is rotatable in the vertical direction. By rotating the intermediate plate 43 upward, the sheets P disposed on the intermediate plate 43 become urged against with the feed roller 15.

When there are one or more sheets P in the sheet cassette 40, the flag 41 b is positioned such that the light receiving component of the sheet sensor portion 41 a is in a light receivable state, as shown in FIG. 8A. In contrast, when there are no sheets on the intermediate plate 43, the flag 41 b is rotated downward as shown in FIG. 8B so as to be moved to a position where the flag 41 b blocks the light receiving component. Thus, the sheet sensor 41 (i.e. the sheet sensor portion 41 a) can determine that the sheet cassette 40 is empty.

Referring back to FIG. 7, if the sum of the distance from the developing portion 110 a positioned farthest from the secondary transfer unit 13 to the end of a primary transfer unit and the distance between the primary transfer unit and the secondary transfer unit 13 is greater than the distance between the secondary transfer unit 13 and the sheet sensor 41, a toner image to be transferred onto the sheet P is formed prior to the feeding of the sheet P so that the leading end of the sheet P can be aligned with the leading end of the toner image.

Therefore, until the last sheet P is fed and the sheet sensor 41 subsequently determines that the sheet cassette 40 is empty, an image forming operation is performed even though there are no sheets remaining in the sheet cassette 40. In other words, at a point where the sheet sensor 41 determines that the sheet cassette 40 is empty, a subsequent toner image is already formed.

Since there are no sheets remaining in the sheet cassette 40, the formed toner image is collected by drum cleaners 18 a to 18 d or by an intermediate-transfer-belt cleaner 19 without being transferred onto a sheet. This leads to a waste of toner and an increase in workload of the cleaners.

Japanese Patent Laid-Open No. 2003-237986 introduces a technique for reducing such waste of toner. Specifically, the presence of sheets is detected by reversing a sheet disposed on a sheet tray in a direction opposite to the sheet feeding direction using a reverse roller. However, this structure requires a reverse roller in addition to a feed roller. Moreover, the sheet feeding process may possibly be retarded since each sheet is reversed by the reverse roller in the direction opposite to the sheet feeding direction.

Therefore, it would be desirable to provide an image forming apparatus that can achieve a less waste of toner and a less workload on cleaners with a simple structure.

SUMMARY OF THE INVENTION

The present invention provides an image forming apparatus that can achieve a less waste of toner and a less workload on cleaners with a simple structure.

According to one aspect of the present invention, an image forming apparatus is provided which includes a sheet container configured to hold therein one or more sheets; a feeder unit configured to feed the one or more sheets from the sheet container; an image-forming unit configured to perform an image forming operation on the one or more sheets fed by the feeder unit; a sheet sensor configured to detect whether or not the one or more sheets are present in the sheet container; and a controller configured to control the image forming operation of the image-forming unit in accordance with a detection result of the sheet sensor, wherein a detection position of the sheet sensor is a trailing end portion of sheet at a held position in the sheet container, and wherein when one sheet of the one or more sheets is to be fed to the image-forming unit, the feeder unit preliminarily conveys the sheet in a sheet feeding direction by a predetermined distance from the held position in the sheet container so that a trailing end of the sheet passes the detecting position of the sheet sensor.

According to another aspect of the present invention, when the feeder unit conveys a last sheet of the one or more sheets in the sheet feeding direction by the predetermined distance from the held position and the sheet sensor thus determines that the sheet container will become empty, the controller controls the image-forming unit so as to stop the image-forming unit from performing a subsequent image forming process that follows a current image forming process for forming a toner image used for forming an image on the last sheet.

According to another aspect of the present invention, the feeder unit includes a feed roller configured to feed the one or more sheets held in the sheet container, and a separator configured to separate each sheet from the one or more sheets fed by the feed roller in a one-by-one fashion and to convey the separated sheet, wherein, as viewed in the sheet feeding direction, if a distance between a leading end of the one or more sheets held in the sheet container and the separator is defined as D and a distance between the separator and the leading end of each sheet conveyed by the predetermined distance is defined as E, the detecting position of the one sheet sensor is located closer towards the leading end of the one or more sheets held in the sheet container from a position distant from the trailing end of the one or more sheets held in the sheet container by D, but is located closer towards the trailing end from a position distant from the trailing end by (D+E).

According to yet another aspect of the present invention, the image forming apparatus may further include a trailing-end regulation member provided in the sheet container and movable in the sheet feeding direction, the trailing-end regulation member regulating the position of the trailing end of the one or more sheets held in the sheet container in accordance with the size of the one or more sheets, wherein the sheet sensor is provided in the trailing-end regulation member.

Moreover, according to yet another aspect of the present invention, the sheet sensor may include a plurality of sheet detection sensors, and wherein the number of the sheet detection sensors provided corresponds to sizes of the one or more sheets that are containable in the sheet container. Additionally, according to still yet another aspect of the present invention, the detection result of the sheet sensor is sent to the controller wirelessly or by electromagnetic induction.

And, according to another aspect of the present invention, an image forming apparatus is provided which includes a first and second sheet containers each holding therein one or more sheets; a first and second feeder units each configured to respectively feed the one or more sheets held in each of the first and second sheet containers; an image-forming unit configured to perform an image forming operation on the one or more sheets fed by the first and second feeder units; a sheet sensor configured to detect whether or not the one or more sheets are present in the first sheet container; and a controller configured to control a sheet feeding process of the first and second feeder units in accordance with a detection result of the sheet sensor, wherein when one sheet of the one or more sheets in the first sheet container is to be fed to the image-forming unit, the first feeder unit preliminarily conveys the sheet in a sheet feeding direction by a predetermined distance from a held position in the first sheet container so that a trailing end of the sheet passes a detecting position of the sheet sensor, and wherein when the first feeder unit conveys a last sheet of the one or more sheets in the first sheet container in the sheet feeding direction by the predetermined distance from the held position and the sheet sensor thus determines that the first sheet container will become empty, the controller controls the second feeder unit so as to switch from the first sheet container to the second sheet container.

Furthermore, according to yet another aspect of the present invention, the image forming apparatus may further include trailing-end regulation members respectively provided in the first and second sheet containers and movable in the sheet feeding direction, each of the trailing-end regulation members regulating the position of the trailing end of the one or more sheets held in the corresponding sheet container in accordance with the size of the one or more sheets, wherein the sheet sensor is provided in the trailing-end regulation member of the first sheet container.

Also, according to another aspect of the present invention, the first feeder unit includes a feed roller configured to feed the one or more sheets held in the first sheet container, and a separator configured to separate each sheet from the one or more sheets fed by the feed roller in a one-by-one fashion and to convey the separated sheet, wherein, as viewed in the sheet feeding direction, if a distance between a leading end of the one or more sheets held in the first sheet container and the separator is defined as D and a distance between the separator and the leading end of each sheet conveyed by the predetermined distance is defined as E, the detecting position of the sheet sensor is located closer towards the leading end of the one or more sheets held in the first sheet container from a position distant from the trailing end of the one or more sheets held in the first sheet container by D, but is located closer towards the trailing end from a position distant from the trailing end by (D+E).

According to the present invention, it can be determined ahead of time that the sheet container will become empty.

Further features and aspects of the present invention will become apparent from the following description of exemplary embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a color image forming apparatus, which is an example of an image forming apparatus according to an exemplary embodiment of the present invention.

FIG. 2 schematically illustrates an exemplary sheet cassette provided in the color image forming apparatus.

FIG. 3 schematically illustrates an exemplary sheet sensor provided in the sheet cassette.

FIGS. 4A and 4B show an exemplary detection process of the sheet sensor for determining whether or not there are any sheets in the sheet cassette.

FIG. 5 shows an alternative example of a sheet cassette provided in the color image forming apparatus.

FIG. 6 shows another alternative example of a sheet cassette provided in the color image forming apparatus.

FIG. 7 schematically illustrates a conventional image forming apparatus.

FIGS. 8A and 8B show a detection process of a conventional sheet sensor for determining whether or not there are any sheets in a sheet cassette.

FIG. 9 shows an exemplary sheet feeding process of the sheet cassette in the color image forming apparatus according to the exemplary embodiment of the present invention.

FIG. 10 shows a modified example of the color image forming apparatus according to the present invention.

DESCRIPTION OF THE EMBODIMENTS

Numerous exemplary embodiments of the present invention will now be described with reference to the drawings.

FIG. 1 schematically illustrates a color image forming apparatus, which is an example of an image forming apparatus according to an exemplary embodiment of the present invention. The image forming apparatus includes an image-forming unit 1, which includes image-forming device 1A to 1D respectively for four colors, which are yellow (y), magenta (M), cyan (C), and black (Bk). The image-forming device 1A to 1D respectively include photoconductive drums 10 a to 10 d, primary charge rollers 2 a to 2 d, exposure portions 3 a to 3 d, developing portions 11 a to 11 d, primary transfer rollers 53 a to 53 d, and cleaners 4 a to 4 d.

The image forming apparatus also includes a sheet cassette 8 defining a sheet container capable of holding sheets P to be fed, and a sheet feeder unit 5 configured to feed each sheet P contained in the sheet cassette 8. The sheet feeder unit 5 includes a feed roller 81 and a pair of separating-conveying rollers 82.

The image forming apparatus further includes an intermediate transfer unit 50, which is provided with an intermediate transfer belt 58 wound around a driving roller 51, a tension roller 52, and a secondary inner transfer roller 56. The intermediate transfer belt 58 can be driven in a direction indicated by an arrow in the drawing. A belt cleaner 55 is provided for cleaning the intermediate transfer belt 58.

Due to an electrostatic bias and a predetermined pressure applied by the primary transfer rollers 53 a to 53 d, a toner image formed on each of the photoconductive drums 10 a to 10 d is primarily transferred to the intermediate transfer belt 58. Subsequently, the toner image primarily transferred on the intermediate transfer belt 58 is secondarily transferred onto one of the sheets P. Specifically, the intermediate transfer belt 58 allows the sheet P to be given an electrostatic bias and a predetermined pressure at a secondary transfer unit 54, which includes the secondary inner transfer roller 56 and a secondary outer transfer roller 57 that substantially face each other. Thus, the unfixed toner image is attached to the sheet P.

A fixing unit 7 includes a rotatable fixing roller 71 and a pressure roller 72 that is rotatable while being urged against with the fixing roller 71. The fixing roller 71 contains therein a heater 73, such as a halogen lamp. The surface temperature of the fixing roller 71 can be adjusted by controlling the voltage applied to the heater 73.

In order to perform an image forming operation in this color image forming apparatus, the surfaces of the photoconductive drums 10 a to 10 d are uniformly charged by the primary charge rollers 2 a to 2 d. The exposure portions 3 a to 3 d perform an exposure process on the photoconductive drums 10 a to 10 d rotating in a direction of an arrow shown in the drawing so as to form an electrostatic latent image on each photoconductive drum.

Subsequently, each of the developing portions 11 a to 11 d performs a toner development process on the electrostatic latent image formed on the corresponding photoconductive drum, thereby forming a toner image on the photoconductive drum. The primary transfer rollers 53 a to 53 d then apply a predetermined pressure and an electrostatic bias against the intermediate transfer belt 58. Thus, the toner image on each photoconductive drum is transferred onto the intermediate transfer belt 58. The transfer toner remaining on the photoconductive drums is collected by the cleaners 4 a to 4 d so as to prepare for a subsequent image forming operation.

An image forming process by the Y, M, C, and Bk image-forming device 1A to 1D of the image-forming unit 1 is implemented by superimposing a toner image over the previous toner image primarily transferred on the intermediate transfer belt 58 by an upstream image-forming unit at a predetermined timing. As a result, a full-color toner image is formed on the intermediate transfer belt 58.

The sheet P is fed by the feed roller 81 in accordance with a predetermined image-forming timing of the image-forming unit 1, and is then conveyed to a resist unit 86 by the pair of separating-conveying rollers 82. After timing adjustment and skew adjustment performed by the resist unit 86, the sheet P is conveyed to the secondary transfer unit 54 defined by the secondary inner transfer roller 56 and the secondary outer transfer roller 57. In the secondary transfer unit 54, the full-color toner image is secondarily transferred onto the sheet P. The transfer toner remaining on the intermediate transfer belt 58, for example, is removed and collected by the belt cleaner 55.

The sheet P having the toner image secondarily transferred thereon is conveyed to the fixing unit 7. In the fixing unit 7, when the sheet P passes through a nip between the pressure roller 72 and the fixing roller 71 rotating at a constant rate, the sheet P is pressed and heated from opposite sides with substantially the same pressure and at substantially the same temperature. Thus, the unfixed toner image on the front face of the sheet P is melted and fixed, thereby forming a color image on the sheet P. Subsequently, the sheet P having the color image formed thereon is ejected outward by ejection rollers 90.

Still referring to FIG. 1, the sheet cassette 8 is provided for holding the sheets P therein. The sheet cassette 8 is provided with a sheet tray T on which the sheets P are stacked, and a trailing-end regulation plate (trailing-end regulation member) 83 that is movable in a sheet feeding direction indicated by an arrow A. When the sheets P are set in the sheet cassette 8, the trailing-end regulation plate 83 may be moved in accordance with the size of the sheets P so as to regulate the position of the trailing end of the sheets P stacked on the sheet tray T.

The feed roller 81 can feed the sheets P stacked on the sheet tray T. The pair of separating-conveying rollers 82 defining a sheet separator separates each sheet from the sheets fed by the feed roller 81 in a one-by-one fashion and conveys only an uppermost sheet. The pair of separating-conveying rollers 82 includes a conveying roller 82 a (FIG. 2) rotatable in the sheet feeding direction, and a separating roller 82 b rotatable in a direction opposite to the conveying direction.

A controller 60 is configured to control, for example, the image forming operation of the image-forming unit 1. A sheet sensor 84 is configured to detect whether there are any sheets P in the sheet cassette 8. In this exemplary embodiment, the sheet sensor 84 is provided in the trailing-end regulation plate 83. Consequently, since the sheet sensor 84 is provided in the trailing-end regulation plate 83, the sheet sensor 84 can constantly detect the presence of sheets P in the sheet cassette 8, that is, the trailing end portion of the sheets P at a held position in the sheet cassette 8, regardless of the size of the sheets P contained in sheet cassette 8. Moreover, such detecting capability can be achieved with a single sheet sensor 84.

In this exemplary embodiment, when an uppermost sheet P contained in the sheet cassette 8 is to be fed as shown in FIG. 2 to perform an image forming operation, the feed roller 81 conveys the sheet P to the pair of separating-conveying rollers 82. Subsequently, the sheet P is temporarily stopped.

Referring to FIG. 2, in a state where a sheet P is contained in the sheet cassette 8, the distance between a leading end of the sheet P in the feeding direction and a nip section (separation point) 82 n between the pair of separating-conveying rollers 82 is indicated by D. On the other hand, in a temporarily stopped state of a sheet P, the maximum distance by which the sheet P is allowed to protrude from the nip section 82 n of the pair of separating-conveying rollers 82 is indicated by E. In other words, when performing an image forming operation, the sheet feeder unit 5 preliminarily feeds the uppermost sheet P by a predetermined distance of (D+E).

Still referring to FIG. 2, the sheet sensor 84 is disposed at a predetermined position which is within a distance range of (D+E) from a rear wall of the trailing-end regulation plate 83. More specifically, within the range of (D+E), the sheet sensor 84 is positioned closer towards the leading end of the sheets P from a position distant from the rear wall of the trailing-end regulation plate 83 by distance D. In other words, a detection point at which the sheet sensor 84 detects the presence of the sheets P is between the position distant from the rear wall of the trailing-end regulation plate 83 by distance D and the position distant from the rear wall of the trailing-end regulation plate 83 by distance (D+E).

In a case where the last sheet P is preliminarily conveyed in the sheet feeding direction by distance (D+E), the sheet sensor 84 does not detect the trailing end of the last sheet P. In a case where the last sheet P is preliminarily conveyed in the sheet feeding direction by distance (D+E) from the held position in the sheet cassette 8, a trailing end of the last sheet passes the detecting point of the sheet sensor 84. Therefore, at a point where the sheet P is preliminarily conveyed in the sheet feeding direction by distance (D+E), the sheet sensor 84 can determine that there are no sheets remaining on the sheet tray T.

In FIG. 2, opposite sides of the sheets P extending parallel to the sheet feeding direction are respectively regulated by a pair of side regulation plates 85. Only one of the side regulation plates 85 is shown in FIG. 2. The side regulation plates 85 and the sheet tray T are rotatable together in the vertical direction. By moving the sheet tray T upward, the sheets P stacked on the sheet tray T become urged against the feed roller 81.

Still referring to FIG. 2, in the exemplary embodiment of the present invention, a slidable contact 89 is provided. Power transmission to the sheet sensor 84 and communication between the sheet sensor 84 and the controller 60 are performed via the slidable contact 89.

FIG. 3 illustrates an exemplary structure of the sheet sensor 84. A reflective optical sensor provided with a light emitting component 841 and a light receiving component 842 is used as the sheet sensor 84. If a light beam L1 emitted from the light emitting component 841 of the sheet sensor 84 is reflected by a sheet P, and a reflected light beam L2 is then received by the light receiving component 842, the sheet sensor 84 determines that one or more sheets P are present. In contrast, if a reflected light beam L2 is not received, the sheet sensor 84 determines that the sheet cassette 8 is empty.

FIGS. 4A and 4B show an exemplary detection process of the sheet sensor for determining whether or not there are any sheets in the sheet cassette. As the sheets P in the sheet cassette 8 are fed sequentially in the course of an image forming operation and are reduced to the very last sheet P1, the sheet sensor 84 determines that the sheet cassette 8 is empty since the sheet sensor 84 does not detect the last sheet P1 when the sheet P1 is in a temporarily stopped state. At this point, the sheet sensor 84 sends empty-cassette information to the controller 60. When the sheet feeding process for the last sheet P1 starts, the sheet cassette 8 becomes completely empty.

In response the empty-cassette information output from the sheet sensor 84, the controller 60 controls the image-forming unit 1 so as to stop the image-forming unit 1 from performing a subsequent image forming process, which follows the current image forming process for forming toner images to be transferred onto the last sheet P1. In other words, with respect to electrostatic latent images formed on the photoconductive drums 10 a to 10 d, a toner development process by the developing portions 11 a to 11 d and a primary transferring process on the intermediate transfer belt 58 are stopped. If the subsequent image forming process is not started yet, the controller 60 prohibits the image forming process.

As described above, the sheet sensor 84 is disposed at a position where it does not detect the trailing end of a sheet P that is preliminarily conveyed in the sheet feeding direction by the predetermined distance (D+E). Accordingly, when the last sheet P1 is preliminarily conveyed by the predetermined distance by the feed roller 81 and the pair of separating-conveying rollers 82 (that is, when the leading end of the last sheet P1 protrudes from the nip section 82 n of the pair of separating-conveying rollers 82 in the sheet feeding direction by the distance E), the sheet sensor 84 can send information to the controller 60 to notify the controller 60 that the sheet cassette 8 will become empty. By sending such information to the controller 60 beforehand, the controller 60 can stop any undesired image forming processes from being performed ahead of time.

The feed roller 81 conveys not only the uppermost sheet P in the sheet feeding direction, but also a second sheet P. However, the second sheet onward are intercepted by the pair of separating-conveying rollers 82. FIG. 9 illustrates a state where two sheets are remaining in the sheet cassette 8, and a second-from-last sheet PS is in a temporarily stopped state while protruding from the nip section 82 n of the pair of separating-conveying rollers 82 by distance E. Since the leading end of the last sheet PL is intercepted by the pair of separating-conveying rollers 82, the trailing end of the last sheet PL is disposed at a position distant from the rear wall of the trailing-end regulation plate 83 by distance D. As mentioned above, within the range of (D+E), the sheet sensor 84 is positioned further towards the leading end of the sheets from the position distant from the rear wall of the trailing-end regulation plate 83 by distance D. Therefore, the sheet sensor 84 determines that the last sheet PL is present.

As described above, the sheet sensor 84 is disposed at a position where the sheet sensor 84 is capable of detecting the trailing end of a sheet P when the sheet P is contained in the sheet cassette 8. Furthermore, by preliminarily conveying the last sheet from the sheet cassette 8 by a predetermined distance before feeding the sheet to the image-forming unit 1, the sheet sensor 84 can send information to the controller 60 to notify the controller 60 ahead of time that the sheet cassette 8 will become empty. Consequently, when the sheet cassette 8 actually becomes empty, the image forming operation can be quickly stopped, thereby contributing to a less waste of toner and a less workload on the cleaners.

In the above description, when the information indicating that the sheet cassette 8 will become empty is quickly sent to the controller 60, the controller 60 stops any undesired image forming processes from being performed beforehand.

Alternatively, in a case where the image forming apparatus is provided with a plurality of sheet cassettes 8, a sheet cassette 8 currently used for feeding the sheets P to the image-forming unit 1 may be switched to another sheet cassette 8. In detail, referring to FIG. 10, in a case where the first sheet cassette 8 becomes empty, the controller 60 may control the sheet feeder unit 5 so that sheets are fed from a second sheet cassette 8A similar to that of the first sheet cassette 8. The second sheet cassette 8A and a second sheet feeder unit 5A shown in FIG. 10 have the same structure as the sheet cassette 8 and the sheet feeder unit 5, respectively. Similar to the above, this contributes to a less waste of toner and a less workload on the cleaners.

And in the alternative, instead of using the slidable contact 89, the power transmission to the sheet sensor 84 and the communication between the sheet sensor 84 and the controller 60 may be performed wirelessly or by electromagnetic induction, as shown in FIG. 5.

In the above description, the pair of separating-conveying rollers 82 are used as an example of a separator for separating each sheet from the stack of sheets in a one-by-one fashion. Alternatively, the separator may be defined by a mechanism that includes a rotary member that is rotatable in the sheet feeding direction and a separating member that is urged against the rotary member.

Furthermore, as an alternative to the sheet sensor 84 that is integrated with the trailing-end regulation plate 83 in the above description, a plurality of sheet sensors 84 a, 84 b, 84 c may be provided as shown in FIG. 6. In that case, the number of sheet sensors provided may correspond to different sizes of sheets P that are containable in the sheet cassette 8. Moreover, each of the sheet sensors may be disposed at a position where the sheet sensor does not detect the trailing end of the last sheet P1 conveyed by a predetermined distance of (D+E).

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all modifications, equivalent structures and functions.

This application claims the benefit of Japanese Application No. 2005-159988 filed May 31, 2005, which is hereby incorporated by reference herein in its entirety. 

1. An image forming apparatus comprising: a sheet container configured to hold therein one or more sheets; a feeder unit configured to feed the one or more sheets from the sheet container; an image-forming unit configured to perform an image forming operation on the one or more sheets fed by the feeder unit; a sheet sensor configured to detect whether or not the one or more sheets are present in the sheet container; and a controller configured to control the image forming operation of the image-forming unit in accordance with a detection result of the sheet sensor, wherein a detection position of the sheet sensor is a trailing end portion of sheet at a held position in the sheet container, and wherein when one sheet of the one or more sheets is to be fed to the image-forming unit, the feeder unit preliminarily conveys the sheet in a sheet feeding direction by a predetermined distance from the held position in the sheet container so that a trailing end of the sheet passes the detecting position of the sheet sensor.
 2. The image forming apparatus according to claim 1, wherein when the feeder unit conveys a last sheet of the one or more sheets in the sheet feeding direction by the predetermined distance from the held position and the sheet sensor thus determines that the sheet container will become empty, the controller controls the image-forming unit so as to stop the image-forming unit from performing a subsequent image forming process that follows a current image forming process for forming a toner image used for forming an image on the last sheet.
 3. The image forming apparatus according to claim 1, wherein the feeder unit includes a feed roller configured to feed the one or more sheets held in the sheet container, and a separator configured to separate each sheet from the one or more sheets fed by the feed roller in a one-by-one fashion and to convey the separated sheet, wherein, as viewed in the sheet feeding direction, if a distance between a leading end of the one or more sheets held in the sheet container and the separator is defined as D and a distance between the separator and the leading end of each sheet conveyed by the predetermined distance is defined as E, the detecting position of the sheet sensor is located closer towards the leading end of the one or more sheets held in the sheet container from a position distant from the trailing end of the one or more sheets held in the sheet container by D, but is located closer towards the trailing end from a position distant from the trailing end by (D+E).
 4. The image forming apparatus according to claim 1, further comprising a trailing-end regulation member provided in the sheet container and movable in the sheet feeding direction, the trailing-end regulation member regulating the position of the trailing end of the one or more sheets held in the sheet container in accordance with the size of the one or more sheets, wherein the sheet sensor is provided in the trailing-end regulation member.
 5. The image forming apparatus according to claim 1, wherein the sheet sensor comprises a plurality of sheet detection sensors, and wherein the number of the sheet detection sensors provided corresponds to sizes of the one or more sheets that are containable in the sheet container.
 6. The image forming apparatus according to claim 1, wherein the detection result of the sheet sensor is sent to the controller wirelessly or by electromagnetic induction.
 7. An image forming apparatus comprising: a first and second sheet containers each holding therein one or more sheets; a first and second feeder units each configured to respectively feed the one or more sheets held in each of the first and second sheet containers; an image-forming unit configured to perform an image forming operation on the one or more sheets fed by the first and second feeder units; a sheet sensor configured to detect whether or not the one or more sheets are present in the first sheet container; and a controller configured to control a sheet feeding process of the first and second feeder units in accordance with a detection result of the sheet sensor, wherein a detection position of the sheet sensor is a trailing end portion of sheet at a held position in the sheet container, wherein when one sheet of the one or more sheets in the first sheet container is to be fed to the image-forming unit, the first feeder unit preliminarily conveys the sheet in a sheet feeding direction by a predetermined distance from the held position in the first sheet container so that a trailing end of the sheet passes the detecting position of the sheet sensor, and wherein when the first feeder unit conveys a last sheet of the one or more sheets in the first sheet container in the sheet feeding direction by the predetermined distance from the held position and the sheet sensor thus determines that the first sheet container will become empty, the controller controls the second feeder unit so as to switch from the first sheet container to the second sheet container.
 8. The image forming apparatus according to claim 7, further comprising trailing-end regulation members respectively provided in the first and second sheet containers and movable in the sheet feeding direction, each of the trailing-end regulation members regulating the position of the trailing end of the one or more sheets held in the corresponding sheet container in accordance with the size of the one or more sheets, wherein the sheet sensor is provided in the trailing-end regulation member of the first sheet container.
 9. The image forming apparatus according to claim 7, wherein the first feeder unit includes a feed roller configured to feed the one or more sheets held in the first sheet container, and a separator configured to separate each sheet from the one or more sheets fed by the feed roller in a one-by-one fashion and to convey the separated sheet, wherein, as viewed in the sheet feeding direction, if a distance between a leading end of the one or more sheets held in the first sheet container and the separator is defined as D and a distance between the separator and the leading end of each sheet conveyed by the predetermined distance is defined as E, the detecting position of the sheet sensor is located closer towards the leading end of the one or more sheets held in the first sheet container from a position distant from the trailing end of the one or more sheets held in the first sheet container by D, but is located closer towards the trailing end from a position distant from the trailing end by (D+E). 